1. Field of the Invention
The present invention relates to network systems for establishing data communication between a plurality of networks such as internetworks and more particularly, to an implementation of a network system which has a function of suitably updating routing information on the decision of communication path and can efficiently manage a minimum amount of routing information.
2. Description of the Related Art
FIG. 7 shows a logical configuration of this sort of conventional network system referred to the above. In the drawing, terminal devices E11, E12, E21 . . . are connected to networks A, B, and C. A manager unit R11 is connected to the networks A and B to transfer data between these networks the shortest route. On the other hand, a manager unit R12 is connected to the networks A and C to transfer data between these networks by the shortest route.
For example, when it is desired to transmit data from the terminal device E11 to the terminal device E21, the terminal device E11 asks the manager unit R11 to relay and transmit the data to the terminal device E21 through the manager unit R11 the shortest route between the networks A and B. in this way, for the purpose of transmitting data by the shortest route, each of the terminal devices previously stores therein the shortest routes from the network connected to its own to the other networks. When data transmission is required, the associated terminal device selects one of the shortest routes according to its destination terminal device, and asks one of the manager units which provides the selected shortest route to relay and transmit the data. The manager units find the shortest routes between the respective networks and broadcast the data as routing information from the manager units to the respective terminal devices. Each terminal device stores the routing information in a routing table located therein. Since the logical configuration of a network system varies from time to time, the routing information is updated as necessary and periodically broadcast to the terminal devices. Accordingly, each terminal device periodically receives the latest routing information and stores it in the table for management.
If the terminal device cannot receive such broadcast routing information for some reason for a period of time, then the information already stored in the routing table of the terminal device is not updated, i.e., becomes old and loses a value as information. In other words, the terminal device continues to possess the old information.
In order to update the routing information held in each terminal device, there has been suggested such a routing information updating system in which each terminal device per se issues an update request on the routing information (refer to Japanese Patent Application Laid-Open Publication No. 2-143759).
However, this system of the above Publication, which is arranged to hold all routing information, is defective in that, when the network system is enlarged in scale or made complicated, this undesirably leads to a very large amount of routing information. For this reason, the prior art system has had a problem that it becomes difficult for each terminal device to secure a memory area enough to store the routing information and management cost becomes high.
Also, in Japanese Patent Application Laid-Open Publication No. 2-143746, there has been suggested a routing information managing system in which each terminal device extracts a necessary route from routing information transmitted from a manger and stores only the extracted route therein for management. This processing system is advantageous in that an information memory area can be reduced since the terminal device is required to store therein only the necessary route extracted from the routing information, but is disadvantageous in that the terminal device still holds the old useless information because the terminal device cannot judge the validation of the information already held therein.